Debrushing felling heads are used in many circumstances, for removing brush on a brush covered terrain. The felling head is usually installed on a tractor vehicle, and controlled by the vehicle driver.
A first common application of the debrushing felling heads is in tree plantations, e.g. pine or cedar plantations. The tree rows therein are laterally spaced from one another by about six feet, and the brush will often reach between one foot and four feet in height, and in the latter case can become higher than the growing trees themselves. Of course, this is highly undesirable, since it then hampers significantly the growth of the trees due to the lack of sun light reaching the trees. The motorized debrushing vehicle is destined to be driven between the tree rows to cut down the undesirable brush which grows there.
Usually, manual means are used for removing the brush located between the trees of a same row, where the debrushing vehicle could not reach without damaging or killing the trees, and thus the finishing job is accomplished by hand once the main debrushing operation is done between the rows by the vehicle.
Debrushing is also accomplished in remote locations, such as along highways or power line paths to maintain the right-of-way in these locations, at regular spaced-apart time intervals. A debrushing vehicle can then become very useful, since in such large areas, manual debrushing is out of the question, being too time-consuming.
It is known in the debrushing industry to use debrushing or felling heads installed on tractor vehicles to accomplish this task. These debrushing heads are often equipped with a rotatable circular saw blade which depends from a frame installed at the front end of the tractor vehicle and which is movable relative thereto due to a boom and arm assembly holding the blade. Thus, by selectively moving the blade transversely across the brush, debrushing of the terrain can be accomplished.
Such a brush cutter is shown in U.S. Pat. No. 4,769,977 issued in 1988 to D. E. Milbourn. The brush cutter workhead is installed at the frontward free end of an arm pivotally hinged upon a boom, the latter being hingedly secured to the tractor vehicle. Hydraulic cylinders control the movement and position of the workhead at all times. The workhead comprises inter alia a circular saw blade, the rotating axis of which can be oriented in a rather wide range of angles to allow horizontal, vertical or inclined cutting paths through the brush. However, this workhead cannot be moved sideways relative to the tractor vehicle. Moreover, the operator has to continuously move the workhead to compensate for uneven ground surfaces when debrushing larger areas, to prevent the saw blade from being damaged by engaging the earth, rocks or other natural ground materials with its cutting edge. Although the shroud provided around the blade has walls which project downwardly under the saw blade level, these thin walls are likely to cut through the ground under the weight of the workhead and the force applied thereon by the hydraulic system if forced against the ground, thus effectively allowing the saw blade to reach the ground level and be damaged during use.
This last disadvantage has been partly solved in U.S. Pat. No. 4,468,917 issued in 1984 to P.-G. Mellgren. The brush cleaning apparatus shown in this patent has a frontwardly projecting workhead equipped with a rotatable circular saw blade, supported at the frontward free end of a work arm. The debrushing device of Mellgren also comprises a very large frame installed over and around the tractor vehicle, and pivoted at the approximate center of gravity thereof for vertical displacement of the saw blade. A rounded skid hub downwardly depends under the saw blade axis, to prevent accidental ground engagement of the blade by a sliding engagement of the hub on the ground surface during the tractor vehicle course. This is true for an even flat ground surface.
However, due to the fact that the rounded semi-spherical hub has a single contact point on the ground, if the ground under the blade becomes suddenly uneven or inclined, the blade may be allowed to accidentally engage the ground and damage itself. Indeed, the single ground-contact point hub does not compensate ground sloping, but only height differences of the ground under the blade, upon sliding displacement of the hub over ground.
The Mellgren patent further provides horizontal pivot means linking the work arm assembly to the tractor vehicle, to allow horizontal pivotal displacement of the workhead, so as to confer a moderately wide access range to the saw blade. This is an improvement, when compared to the previously mentioned Milbourn patent.
The Mellgren patent discloses a single saw blade, as with the Milbourn patent.
Canadian patent No. 1,135,599 issued in 1982 to J. Denis shows a pair of circular saw blades installed in side-by-side configuration, and a cover for preventing high velocity airborne projectiles issuing from the blades to endanger the immediate environment around the rotating blades. The two saw blades rotate in opposite directions, so as to draw the brush between them and then accomplish a shearing effect thereon, combined with the cutting properties of each blade. The blades rotational axes are slightly longitudinally offset relative to the direction of travel of the vehicle, so as to allow them to be slightly inwardly offset towards one another. A radial play has to exist between the two coplanar blades, to prevent them from highly undesirable mutual contact during use, and the longitudinal and inward offsets allow the blades to prevent a brush plant or tree from being engaged, along a line parallel to the direction of travel, between the two blades and remain at least partly uncut, due to this play. However, this offset of the blades may result in an unstable engagement of the blades upon larger trees, due to the unequal engagement of the two blades on the tree.
The Denis patent moreover requires the controller to adjust the vertical height of the saw blades relative to the ground continuously, to prevent ground engagement thereof, as with the Milbourn patent.